Relocate the Result type from `@/data/result` to `@/utils/result` and
replace the discriminated-union API (`kind`/`value`) with a simpler
boolean-based API (`success`/`data`) across all repositories and
storage classes for improved readability and consistency.
Replace minimal background/foreground vars with a comprehensive design
system covering colors (background, text, brand, profile, gifts,
activities, nav), spacing, and border-radius tokens. Mirrors the
original Dart design constants under src/design/*.ts and supports
dark mode.